ART: Refactor GenerateTestAndBranch

Each code generator implements a method for generating condition
evaluation and branching to arbitrary labels. This patch refactors
it for better clarity but also to generate fewer jumps when the true
branch is the fallthrough successor.

This is preliminary work for implementing HSelect.

Change-Id: Iaa545a5ecbacb761c5aa241fa69140cf6eb5952f